Fruit and vegetable jams and jellies are normally prepared by cooking protreated fruit and subsequent cooling. The pectin present in the fruit gelates, thereby giving the jam or jelly its rheological characteristics. The pectin is mainly responsible for this gel formation due to its gelifying power. The gelifying potential of pectin is dependent on several conditions, mainly:
dry substance content or brix, PA1 pH of the fruit or vegetable, PA1 concentration of pectin normally present, PA1 degree of methylation of the pectin.
Pectins are major constituents of the cell walls of edible parts of fruits and vegetables. The middle lamella which are situated between the cell walls are mainly build up from protopectin which is the insoluble form of pectin. Pectins are considered as intercellular adhesives and due to their colloid nature they also have an important function in the waterregulation of plants. Waterbinding capacity is greatly increased by the amount of hydrophylic hydroxyl and carboxyl groups. The amount of pectin can be very high. For example, lemon peels are reported to contain pectin upto 30% of their dry weight, orange peels contain from 15-20% and apple peels about 10% (Norz, K., 1985. Zucker und Susswaren Wirtschaft 38 5-6).
From a chemical point of view pectin consist of methoxylated polygalacturonic acid residues. Pectins are classified in differemt categories based on the degree of esterification and the degree of polymerisation.
On the basis of the degree of esterification pectins are divided into two groups:
1) high-methoxylated pectins with a degree of esterification higher than 50%, and
2) low-methoxylated pectins having a degree of esterification lower than 50%.
Both of these groups are capable of forming gels, however these gels differ in the mechanism by which they are formed. The high-methoxylated pectins form gels based on dehydration and electrical neutralisation of colloidal pectin-agglomerates. Gelation is stimulated if the pH is about 3 (dependent on the fruit or vegetable in question) and sugar, needed for the dewatering, is present in more than 60% dry weight. Depending on the fruit, in practice, this often means that both sugar and high-methoxylated pectin have to be added to obtain the classical high sugar content jams. Structure forming interactions in the high-methoxylated pectin containing gels is based on hydrogen bond formation.
The low-methoxylated pectins are capable of forming gels with calcium ions or other divalent cations only. Gel formation with low-methoxylated pectins is based on ionic interactions. Calcium ions are naturally present in for example apples. No sugars have to be added in order to obtain suitable gelling properties. The amount of sugar and the pH in this case only influence the speed and temperature of gelifying. Low-methoxylated pectins are therefore perfectly suited for the preparation of low sugar content jams and jellies.
In practice pectin from fruits and vegetables generally has a high degree of methoxylation which necessitates the addition of large amounts of sugars to obtain suitable rheological characteristics. Furthermore, the degree of methoxylation varies with the time elapsed between the harvesting and the processing of the vegetables or fruit. This leads to a difference in the viscosity of the produced jams and jellies if the processing conditions are kept constant.